Second-Harmonic Generation: A Probe of Symmetry and Order in Crystalline Surfaces

Abstract

Since optical second-harmonic generation (SHG) is (electric-dipole) forbidden within a centrosymmetric medium, the process exhibits a high degree of sensitivity to the character of the medium’s surface, the region where the inversion symmetry of the bulk is broken. This intrinsic surface sensitivity, combined with the high spectral and temporal resolution and wide range of applicability afforded by optical techniques, makes SHG an attractive tool for the study of surfaces and interfaces. To date, the SHG technique has been exploited primarily in investigations of adsorbate-covered surfaces and interfaces [1]. In this paper, we describe the extension of the SHG technique to studies of clean, ordered surfaces, showing that the polarization dependences of the SH signal reflect the symmetry and ordering of the surface atomic structure [2,3]. Results are presented here for Si(111)-2x1 and 7x7 reconstructed surfaces and for Si(111)-7x7 surfaces modified by the room-temperature deposition of Si atoms [4]. Note that all of the materials under study consist of pure Si, both at the surface and in the bulk. We can, nonetheless, readily examine their surface properties because of the intrinsic surface sensitivity provided by the SHG process.